ABSTRACT Acute kidney injury (AKI) occurs in nearly 1 of 5 hospitalized patients and is associated with increased morbidity and mortality across all ages. Many AKI patients will recover kidney function post-injury but then progress to chronic kidney disease (CKD). The mechanisms are poorly understood and there are currently no effective therapies to prevent, limit, or reverse the tissue damage. There is a critical need to identify mechanisms involved in the pathogenesis of AKI. Our long-term goal is to elucidate these mechanisms and leverage them for new therapies to limit AKI and prevent the transition to CKD. Proximal tubule epithelial cells (PTEC), a major site of damage during AKI, are very metabolically active and rich in mitochondria. Mitochondrial metabolism causes increased reactive oxygen species (ROS) which has been implicated in both ischemia-reperfusion injury (IRI) and cisplatin-induced nephrotoxicity. Modulating mitochondrial function during AKI is an attractive, but thus far unachievable, strategy. Our central hypothesis is that loss of the mitochondrial sirtuin lysine deacylase Sirt5 leads to shifts in PTEC metabolism that protect against AKI. This is supported by preliminary data showing protection against both IRI and cisplatin-induced AKI in global Sirt5 knockout (Sirt5-/-) mice in vivo and in vitro as well as in primary human PTEC with siRNA knockdown of Sirt5. Further data support our proposed mechanism of protection in which Sirt5-/- PTEC exhibit a form of metabolic preconditioning characterized by a shift of fatty acid oxidation (FAO) from mitochondria to peroxisomes. Peroxisomes have previously been linked to renoprotection in other animal models, most likely due to their ability to eliminate ROS. In Sirt5-/- kidneys, peroxisome number is increased at baseline and the peroxisomes are more resistant to damage during AKI. Our central hypothesis will be tested with three aims. Aim 1 will define the specific site of Sirt5 action during kidney injury with a particular focus on PTEC. While aim 2 will drill down on the role of peroxisomal fatty acid oxidation during kidney injury. Finally, aim 3 will mechanistically define the molecular targets of Sirt5 during kidney injury. All of the three aims will utilize a rigorous, mechanistic approach that combines in vitro and in vivo models. In vivo studies in mice will use both global Sirt5-/- and inducible, PTEC-specific knockout of Sirt5 as well as global LCAD-/- (key mitochondrial FAO enzyme). In vitro studies will use isolated primary mouse and human PTEC as well as genetically manipulated mouse and human cell lines. Human AKI will be modeled in mice by unilateral ischemia-reperfusion injury and single high dose treatment with the nephrotoxin cisplatin. In both models of injury, the role of Sirt5 in mediating the progression from acute to chronic kidney disease will be studied. This project will significantly advance the field by opening up new therapeutic avenues where Sirt5 can be pharmacologically inhibited in the context of AKI to protect against injury and block the progression to chronic kidney disease.